The present invention relates to a polypropylene resin composition having an improved water content and pre-expanded particles made thereof. More particularly, the present invention relates to a water-containing polypropylene resin composition employed for preparation of pre-expanded particles which can be suitably employed, for example, as a raw material for in-mold foamed articles, and pre-expanded particles made thereof.
As a process to obtain pre-expanded particles of a polypropylene resin, the process has been conventionally known, wherein polypropylene resin particles containing a blowing agent are dispersed in an aqueous dispersion medium, and heated to a temperature not lower than the softening point of the resin with maintaining a pressure in the vessel not less than vapor pressure of the blowing agent, and then released into an atmosphere of lower pressure than that in the vessel to expand the particles (for example, Japanese Unexamined Patent Publication No.77174/1977).
In general, a blowing agent such as a volatile organic blowing agent or carbon dioxide gas has been needed to prepare pre-expanded particles in the process for preparing pre-expanded particles. Since a water content under water vapor pressure at the melting point of polypropylene resin is less than 1% by weight and it is difficult to pre-expand polypropylene as water-containing particles made thereof.
However, the volatile organic blowing agent is not preferable, since propane, butane and the like have problems concerning safety and chlorofluorocarbon and the like have problems in an environmental point of view such as destruction of the ozone layer. Furthermore, it has been known that a volatile blowing agent has difficulty in controlling an expansion ratio.
On the other hand, carbon dioxide gas is not preferably employed from the viewpoint of global warming. And the cost of apparatus becomes high, since it needs a large-scale apparatus to make high pressure during preparation of pre-expanded particles.
As a method to solve the above-mentioned problems, there has been proposed a method by expanding polypropylene resin particles containing 30 to 50% by weight of an inorganic filler by using dispersion medium water as a blowing agent (Japanese Examined Patent Publication No.2183/1974).
However, this method has problems that adhesion between particles is insufficient at in-mold molding and that mechanical strength and flexibility of the obtained articles become low, since a large amount of a filler is employed.
On the other hand, as a method for obtaining polypropylene resin pre-expanded particles by using dispersion medium water as a blowing agent, there has been proposed a method by using a random copolymer of ethylene and propylene having an ethylene content of 4 to 10% by weight and containing 0.4 to 10% by weight of a higher fatty acid metal salt having 12 to 22 carbon atoms as a base resin (Japanese Unexamined Patent Publication No.188435/1985), or a method by using a copolymer of ethylene and propylene having an ethylene content of 2 to 10% by weight as a base resin and adjusting an initial internal pressure in a vessel before heating to 5 kg/cm.sup.2 G with inorganic gas (Japanese Unexamined Patent Publication No.221440/1985).
However, these methods lack productivity and economy, and particle adhesion is easy to occur in a dispersion medium, since they need heating condition at 160.degree. C., for at least 10 hours to obtain pre-expanded particles having a high expansion ratio by using these methods.
Therefore, there has been recently desired a method for preparing polypropylene resin pre-expanded particles having desired properties without blowing agents such as a volatile organic blowing agent and carbon dioxide gas, which have been conventionally needed.
In order to solve the above-mentioned problems, the present inventors developed and filed a method for preparing pre-expanded particles having desired properties without a volatile blowing agent or carbon dioxide gas (Japanese Unexamined Patent Publication No.84124/1996). The method comprises preparing particles of a polypropylene resin and a hydrophilic polymer as base resins, which has an improved water content, dispersing them into an aqueous dispersion medium in an air-tight vessel, heating them at a temperature not lower than the softening point of polypropylene to obtain a water content of 1 to 50% by weight, and releasing them into an atmosphere of low pressure to expand the particles containing water.
By using the pre-expanded particles prepared by the above-mentioned method, a foamed articles having an excellent mechanical strength, heat resistance and water resistance can be obtained in case of a low expansion ratio, in spite of employing no volatile blowing agent or carbon dioxide gas.
However, pre-expanded particles having a higher expansion ratio (an apparent expansion ratio of at least 25 times) are needed in cushioning material use which requires excellent flexibility and cushioning property. Since at least 5 parts by weight of a filler and more than 10 parts by weight of a hydrophilic polymer must be employed based on 100 parts by weight of a polypropylene resin in order to obtain pre-expanded particles having such a high expansion ratio, a foamed article obtained by expanding such pre-expanded particles has problems that the above properties (mechanical strength, heat resistance and water resistance) become lower, they can not be employed in uses requiring excellent properties, and cost for preparation thereof becomes higher.